I. Field of the Invention
The present invention relates generally to wireless communication systems, and more particularly to a system and method for controlling temperature compensated output power of hand-held wireless devices connected to external antenna units. The present invention is most applicable to wireless communications devices transmitting to communication systems using code division multiple access (CDMA) modulation techniques where accurate control of output power is very important.
II. Related Art
Mobile telephone systems allow customers to establish communication links or place telephone calls from wireless devices such as portable or hand-held mobile phones. Calls initiated or received by wireless devices used in such systems are processed by a wireless communications network. One type of wireless network is a terrestrial cellular communication system communicating via a series of base stations and ground-based antennas that operate in the 800-1900 MHz range. Cellular communication systems limit the user to communication within a cell, which comprises a geographical service area to which the base station antennas can transmit. Users can move from cell to cell through known hand-off procedures that transfer calls from one cell to another. However, if no base station is within range of the mobile transmitter, such as in a rural area, a user cannot use the mobile telephone service.
Developments in mobile telephone system technology have led to wireless communication systems or networks that can transfer signals using a Low Earth Orbit (LEO) satellite system. The satellite systems can transmit and receive signals in rural areas as well as cities through the beams they project, and a user does not need to be within close range of a ground-based antenna. As a result, satellite communication systems are not limited to major cities as are cellular networks. In addition, each LEO satellite is capable of carrying a large number of user transmissions simultaneously. Various satellite access schemes such as time division multiple access (TDMA) and code division multiple access (CDMA) allow concurrent access to LEO satellites by a large number of users.
The number of users that can be serviced by a wireless communication system, the system capacity, increases if the power output from each user""s wireless device is decreased to the minimum power needed for quality transmission, and overhead or non-traffic messages or channel activity is reduced. This is the result of decreasing mutual interference between users, which is especially important in environments such as CDMA type communication systems. If one user""s signal is too strong the quality of service for other users degrades due to increased interference. However, if the power of a user""s signal becomes too low, the quality of service for that user becomes unacceptable. So, there is a desire to maintain as high a power level as possible to have higher quality service.
Thus, the number of users that may be provided service is increased by maintaining overhead power levels and each individual user""s signals at the minimum levels needed for optimum performance. Therefore, the power output of wireless device transmissions are generally controlled using one or more power control methods to minimize interference and maximize communication link quality. Techniques for power control are discussed for example in U.S. Pat. No. 5,383,219, entitled xe2x80x9cFast Forward Link Power Control In A Code Division Multiple Access System,xe2x80x9d issued Jan. 17, 1995; U.S. Pat. No. 5,396,516, entitled xe2x80x9cMethod And System For The Dynamic Modification Of Control Parameters In A Transmitter Power Control System,xe2x80x9d issued Mar. 7, 1995; and U.S. Pat. No. 5,267,262, entitled xe2x80x9cTransmitter Power Control System,xe2x80x9d issued Nov. 30, 1993, which are incorporated herein by reference. In addition, see U.S. patent application Ser. No. 09/164,384 filed Sep. 30, 1998 entitled xe2x80x9cSystem And Method For Optimized Power Controlxe2x80x9d; and 08/908,528, filed Aug. 7, 1997, entitled xe2x80x9cMethod And Apparatus For Adaptive Closed Loop Power Using Open Loop Measurements,xe2x80x9d which are incorporated herein by reference. The result is the communication system efficiently carries the substantially maximum number of individual user transmissions simultaneously.
Wireless devices, also referred to as user terminals, in current wireless communications systems may be any of several different types. One type is the portable unit, which is a hand-held device carried by the user and requires no external power source or antenna system. Another type is the mobile unit or station, which is typically fixed in a vehicle and operates like a desk type phone. A mobile unit has a separate unit (or xe2x80x9cboxxe2x80x9d) that is mounted in the vehicle and contains most of the transmitting and receiving circuits or hardware. A hand-held unit such as a phone handset, containing a keypad, speaker and microphone, is connected by one or more cables, conductors, or connectors to the box. A cradle is provided for supporting the handset unit when it is not in operation or when it is being used in a xe2x80x9chands freexe2x80x9d mode. The box in turn is connected by a cable to an externally mounted xe2x80x9coutdoorxe2x80x9d antenna unit, which transmits and receives signals via a satellite or terrestrial cellular communications system or a base station or gateway.
A third type of user terminal combines the features of both a portable unit and a mobile unit. This type uses a hand-held device that can be used as a standalone unit away from the vehicle, and can be connected to a vehicle mounted assembly sometimes called a xe2x80x9ccar kit,xe2x80x9d for use in the vehicle. The car kit uses an external or outdoor unit (ODU) with an outdoor antenna to accommodate communications for the wireless device. A primary advantage of this combination unit or arrangement is that when the wireless device is used in the vehicle it can utilize additional power provided by the vehicle mounted electronics to establish a better and stronger communications link with satellite transceivers. It also allows conservation of internal battery power, drawing on vehicle provided power instead.
Satellite wireless telephone systems are particularly sensitive to outdoor or external antenna unit matching at the mobile unit due to potential path losses and a resulting difficulty in power control. The power output of the outdoor unit is calibrated against, or configured with, specific phone transmission circuits in mind. At present, a given car-kit is designed or calibrated in the factory to function with certain characteristics for phones with which it is to be connected. Once this designing or calibration (matching) takes place, the autonomous nature of phones and car kits is diminished because the phone is limited to being used with certain car kits or outdoor antenna units, such as specific models or manufacturers, having closely matched characteristics, against which it was calibrated. This clearly sets certain constraints for mobile units or hand-held phones used with car kits in order to provide a closer match between the power output desired by the phone power control systems or methods, and the power actually being delivered by the outdoor unit.
A goal of the present invention is to allow a variety of mobile wireless devices to be used with a given vehicle mounted, or fixed, xe2x80x9ccar kitxe2x80x9d or outdoor unit with associated antenna,xe2x80x9d while maintaining a desired level of accuracy for the power output by the outdoor unit. It is not uncommon that a user may have more than one portable phone (called a xe2x80x9chand held unitxe2x80x9d) or have one that can be used both as a standalone unit and as a mobile phone when placed in a cradle mounted in a vehicle. Also, a user may commonly upgrade his or her phone as new models come on the market, and have more than one car-kit. Therefore, the mobile unit employed with a car kit may change permanently or on a transient basis, creating potential undesirable matching problems. For optimum performance, the antenna power must be calibrated with the specific phone with which it is to be used, and here a given antenna unit is not calibrated against a specific hand held device.
What is needed is a system and method for calibration by providing an antenna output power measurement of its information signal to the hand held device. To optimize the performance of the antenna unit, the power measurement sent to the hand held device should be compensated for temperature.
The present invention is a system and method for controlling transmit power in a wireless device that is interfacing with another device, such as embodied in a car kit, that has an external antenna and signal processing or amplification elements coupled to the wireless device. For example, a hand-held wireless phone coupled to a vehicle xe2x80x9ccar kitxe2x80x9d assembly. The antenna unit of the present invention includes components for transmitting the information signal including: an external antenna unit, a cradle, and other elements needed for transmission. With the present invention, transmit power is controlled by obtaining an output power measurement of the information signal transmitting from the antenna unit, also referred to as an outdoor unit. The power measurement is provided to the hand held device via a feedback loop to adjust the initial power of the information signal transmitting from the hand held device. In addition, the power measurement provided to the hand held device is compensated for temperature.
The present invention provides two different schemes for temperature compensation: a software temperature compensation scheme and a hardware temperature compensation scheme.
The outdoor unit of the present invention includes components for transmitting the information signal and components for obtaining an output power measurement of the information signal immediately prior to transmission. The hand held device comprises one or more transmit and receive components and a logic unit. The logic unit of the hand held device comprises software and a transmit power control. The software of the present invention utilizes the temperature compensated power measurement of the outdoor unit to operate the transmit power control in such a way as to optimize the performance of the outdoor unit. The hand held device interprets the output power measurement and adjusts the transmission power of the information signal accordingly.
The software temperature compensation scheme involves identifying a power detector output and temperature output within the outdoor unit and sending both outputs through a power estimator, where the power estimator utilizes voltage-temperature correlation equations, to produce the temperature compensated power measurement.
The hardware temperature compensation scheme involves the use of a temperature compensation circuit made up of a diode pair, an amplifier, a negative temperature coefficient thermistor, and a positive temperature coefficient thermistor. The diode pair is used to provide a true average of the current power input and sends its output to the amplifier. The amplifier produces the temperature compensated power measurement, where the negative temperature coefficient thermistor increases the temperature compensated power measurement at low temperatures and the positive temperature coefficient thermistor increases the temperature compensated power measurement at high temperatures.
The method of the present invention includes obtaining by the antenna unit a temperature compensated power measurement, modulating the temperature compensated power measurement, and transmitting the modulated measurement signal to the hand held device. The method of the present invention also includes receiving the modulated measurement signal by a cradle or hand held device, demodulating the measurement signal, and adjusting by the hand held device the transmit power of the information signal.
The system and method of the present invention allow control of the initial transmit power of the information signal from the hand held device in order to limit the output power of the signal transmitted from the device to the power needed for high quality transmission. Efficient use of power by limiting the output power from the hand held device allows the maximum number of individual user transmissions to be carried by a satellite in a wireless communication system.